Inertia Rails Architecture

Server-driven architecture for Rails + Inertia.js + React when building pages, forms, navigation, or data refresh. Inertia is NOT a traditional SPA — the server owns routing, data, and auth. React handles rendering only.

The Core Mental Model

The server is the source of truth. React receives data as props and renders UI. There is no client-side router, no global state store, no API layer.

Before building any feature, ask:

• Where does the data come from? → If server: controller prop. If user interaction: useState.
• Who owns this state? → If it's in the URL or DB: server owns it (use props). If it's ephemeral UI: React owns it.
• Am I reaching for a React/SPA pattern? → Check the decision matrix below first — Inertia likely has a server-driven equivalent.

Decision Matrix

Need	Solution	NOT This
Page data from server	Controller props	useEffect + fetch
Global data (auth, config)	inertia_share + usePage()	React Context / Redux
Flash messages / toasts	Rails flash + usePage().flash	inertia_share / React state
Form submission	<Form> component	fetch/axios + useState
Navigate between pages	<Link> / router.visit	react-router / window.location
Refresh specific data	router.reload({ only: [...] })	React Query / SWR
Expensive server data	InertiaRails.defer	useEffect + loading state
Infinite scroll	InertiaRails.scroll + <InfiniteScroll>	Client-side pagination
Stable reference data	InertiaRails.once	Cache in React state
Real-time updates (core)	ActionCable + router.reload	Polling with setInterval
Simple polling (MVP/prototyping)	usePoll (auto-throttles in background tabs)	setInterval + router.reload
URL-driven UI state (dialogs, tabs)	Controller reads params → prop, router.get to update	useEffect + window.location
Ephemeral UI state	useState / useReducer	Server props
External API calls	Dedicated API endpoint	Mixing with Inertia props

Rules (by impact)

#	Impact	Rule	WHY
1	CRITICAL	Never useEffect+fetch for page data	Inertia re-renders the full component on navigation; a useEffect fetch creates a second data lifecycle that drifts from props and causes stale UI
2	CRITICAL	Never check auth client-side	Auth state in React can be spoofed; server-side checks are the only real gate. Client-side "guards" give false security
3	CRITICAL	Use <Form>, not fetch/axios	<Form> handles CSRF, redirect-following, error mapping, file detection, and history state — fetch duplicates or breaks all of this
4	HIGH	Use <Link> and router, not <a> or window.location	<a> triggers a full page reload, destroying all React state and layout persistence
5	HIGH	Use partial reloads, not React Query/SWR	React Query adds a second cache layer that conflicts with Inertia's page-based caching and versioning
5b	HIGH	Use usePoll only for MVPs; prefer ActionCable for production real-time	usePoll is convenient but wastes bandwidth — every interval hits the server even when nothing changed. ActionCable pushes only on actual changes
6	HIGH	Use inertia_share for global data, not React Context	Context re-renders consumers on every change; shared props are per-request and integrated with partial reloads
7	HIGH	Use Rails flash for notifications, not shared props	Flash auto-clears after one response; shared props persist until explicitly changed, causing stale toasts
8	MEDIUM	Use deferred/optional props for expensive queries	Blocks initial render otherwise — user sees blank page until slow query finishes
9	MEDIUM	Use persistent layouts for state preservation	Without persistent layout, layout remounts on every navigation — scroll position, audio playback, and component state are lost
10	MEDIUM	Keep React components as renderers, not data fetchers	Mixing data-fetching into components makes them untestable and breaks Inertia's server-driven model

Skill Map

Common workflows span multiple skills — load all listed for complete coverage:

Workflow	Load these skills
New page with props	inertia-rails-controllers + inertia-rails-pages + inertia-rails-typescript
Form with validation	inertia-rails-forms + inertia-rails-controllers
shadcn form inputs	inertia-rails-forms + shadcn-inertia
Flash toasts	inertia-rails-controllers + inertia-rails-pages + shadcn-inertia
Deferred/lazy data	inertia-rails-controllers + inertia-rails-pages
URL-driven dialog/tabs	inertia-rails-controllers + inertia-rails-pages
Alba serialization	alba-inertia + inertia-rails-typescript
Testing controllers	inertia-rails-testing + inertia-rails-controllers

References

MANDATORY — READ ENTIRE FILE before building a new Inertia page or feature: references/AGENTS.md (~430 lines) — full-stack examples for each pattern in the decision matrix above.

MANDATORY — READ ENTIRE FILE when unsure which Inertia pattern to use: references/decision-trees.md (~70 lines) — flowcharts for choosing between prop types, navigation methods, and data strategies.

Do NOT load references for quick questions about a single pattern already covered in the decision matrix above.

When You DO Need a Separate API

Not everything belongs in Inertia's request cycle. Use a traditional API endpoint when:

Signal	Why	Example
Non-browser consumer	Inertia's JSON envelope (component, props, url, version) is designed for the frontend adapter — other consumers can't use it	Mobile API, CLI tools, payment webhooks
Large-dataset search	Dataset is too big to load as a prop; each input needs per-keystroke server filtering. Use raw fetch for the search, let Inertia handle post-selection side effects via props.	City/address autocomplete, postal code lookup
Binary/streaming response	Inertia can only deliver JSON props. Use a separate route with a standard download response.	PDF/CSV export, file downloads